Heretofore, the general sliding bearings of an internal combustion engine are, as is described in Japanese Unexamined Patent Publication (Kolai) No. Sho 60-145,345, provided by sintering a copper-alloy, i.e., the so-called lining, which consists of from approximately 8 to 35% of Pb, not more than approximately 10% of Sn, and the balance consisting of Cu, on a strip consisting of low-carbon steel, such as SAE 1010 and 1020.
Furthermore, as is described in detail in Japanese Patent Divulgation (Kohyo) No. Hei 1-503,150, Sn diffuses into the lining of a bearing structure with a Pb--Sn based or Pb--Sn--Cu based overlay on the lining, with the result that Sn is so depleted in the overlay as to drastically deteriorate the corrosion resistance against the lubricating oil. An Ni barrier has, therefore, been sandwiched by plating between the lining and the overlay. The proposal in the publication mentioned above as a countermeasure against the corrosion attributable to lead resides in the refinement of the lead phases.
The lead, which is contained in the conventional overlay-fitted kelmet bearing, is a soft metal and has an excellent sliding property and conformability. Therefore, lead, which realises the conformability, has been used for the purpose of preventing the seizure due to adhesion of Cu.
When the conventional sliding bearing is used in degraded lubricating oil for extended periods of time, and the lining is then exposed, the lead phases of the lining are corroded and dissolved, thereby roughening the lining surface and hence leading to seizure. Alternatively, since the lead phases are dissolved out, thus forming pores, the strength of the lining is lowered, leading to collapses of the lining, thereby incurring seizure. A measure to refine the lead phases has been undertaken to mitigate the corrosion of lead phases. However, this measure involves limitations arising from the use of lead insofar as the sliding bearing materials contain lead.
In addition, when the conventional sliding bearing is used in degraded lubricating oil for extended periods of time, copper reacts with the sulfur (S) in the lubricating oil and is sulfurized to form copper sulfide on the lining surface. As a result, the corrosion resistance and wear resistance detrimentally deteriorate. As a measure against this deterioration, Zn addition is implemented. However, the addition of Zn virtually has no effect on enhancement of the seizure resistance.
Furthermore, the conventional overlay is 20 .mu.m or more thick so as to not only provide conformability with the shaft but also to exhibit the sliding-bearing performances, such as seizure resistance, due to the overlay itself. It is, however, needless to say that thick plating should be avoided from the point of view of the cost of sliding-bearing materials.
There is a problem from another point of view. That is, although the conventional nickel barrier must be used for preventing the Sn diffusion, when the nickel barrier is exposed due to the wear of overlay, seizure becomes extremely liable to occur on the exposed hard Ni portions. This result has been occasionally judged to be the life of a bearing. Although such problems of the Ni barrier have been pointed out heretofore, it can be said that the use an Ni barrier is inevitable for preventing the tin diffusion.